When does time limit begin?

[John Robertson]

As we all know many tournament games (and some league games) are subject to time limits. Often though, it isn't made clear in league or tournament rules as to when this time actually starts. Over the years I've heard various umpires say they "start their watches"...

1. When the ground rules end
2. When the pitcher finishes his/her warmups
3. When the first batter steps into the batter's box
4. When "play ball" is declared
5. When the first pitch is thrown

There isn't a heck of a lot of time difference between numbers 2 through 5, but you never know when it might make a difference. I'd like to hear umpires' opinions about when they personally "start the clock."